Void and phase evolution during the processing of Bi-2212 superconducting wires monitored by combined fast synchrotron micro-tomography and x-ray diffraction

Recent study of the current-limiting mechanisms in Bi-2212 round wires has suggested that agglomeration of the residual Bi-2212 powder porosity into bubbles of filament-diameter size occurs on melting the Bi-2212 filaments. These pores introduce a major obstacle to current flow, which greatly reduces the critical current density (J(c)). Here we present an in situ non-destructive tomographic and diffraction study of the changes occurring during the heat treatment of wires and starting powder, as well as a room temperature study of ex situ processed wires. The in situ through-process study shows that the agglomeration of residual porosity is more complex than previously seen. Filament changes start with coalescence of the quasi-uniform and finely divided powder porosity into lens-shaped defects at about 850 degrees C when the Bi-2201 impurity phase decomposes before the Bi-2212 starts to melt. These lens-shaped voids grow to bubbles of a filament diameter on melting of the Bi-2212 and continue to lengthen and then to agglomerate across multiple filaments while the filaments are in the liquid state. The experiment makes clear why melt processing is vital to developing high J(c) and also shows how rearrangement of the residual filament porosity on melting imposes a strong longitudinal inhomogeneity in each filament. Reducing the bubble density is clearly an important path to reaching much higher J(c) values in Bi-2212 round wires. Synchrotron micro-tomography is an exceptionally powerful technique for studying the spatial extent of the porosity on a scale of about 2 mu m and larger.